1. Field of the Invention
The present invention relates to a mounting structure for a semiconductor device, and more specifically to a mounting structure for a semiconductor device of high calorific power such as a high power transistor or a microwave monolithic IC (integrated circuit), which has low thermal resistance and high thermal conductivity.
2. Description of Related Art
Semiconductor devices have improved in their output power and operating speeds. In particular, semiconductor devices used for processing microwave signal generally have high output power and rapid operating speeds. These high output and rapid semiconductor devices generate much heat so that their operation properties may be lowered by overheating.
In order to prevent the overheating, the semiconductor device of high calorific power is mounted on a substrate of a material of high thermal conductivity. Namely, the substrate of low thermal resistance allows the heat generated by the semiconductor device to efficiently dissipate. For example, beryllia (BeO) having a high thermal conductivity is used for the substrate.
In case of using a beryllia substrate, both surfaces of the beryllia substrate are metalized and the beryllia substrate is brazed to a base member formed of copper or copper-tungsten alloy. A semiconductor device or devices are mounted on a surface of the beryllia substrate mounted in the base member.
Although beryllia has higher thermal conductivity than other ceramic materials used for the substrate, recently heat radiation of the above mounting structure utilizing beryllia has become insufficient for dissipating heat generated by the semiconductor device. In order to improve the heat radiation of the mounting structure, using a thinner beryllia substrate has been considered.
However, since beryllia is poisonous and has poor processability, it is difficult to thin the beryllia substrate. Thus, the heat radiation of the mounting structure utilizing beryllia substrate can not be improved by using a thinner beryllia substrate. In addition, materials having higher thermal conductivities than beryllia such as diamond and cubic boron nitride (c-BN) are too expensive for the mounting of the semiconductor device.